Catalytic hydrogenation of olefins by a multifunctional molybdenum-sulfur complex

Exploration of molybdenum complexes as homogeneous hydrogenation catalysts has garnered significant attention, but hydrogenation of unactivated olefins under mild conditions are scarce. Here, we report the synthesis of a molybdenum complex, [Cp*Mo(Ph2PC6H4S−CH = CH2)(Py)]+ (2), which exhibits intriguing reactivity toward C2H2 and H2 under ambient pressure. This vinylthioether complex showcases efficient catalytic activity in the hydrogenation of various aromatic and aliphatic alkenes, demonstrating a broad substrate scope without the need for any additives. The catalytic pathway involves an uncommon oxidative addition of H2 to the cationic Mo(II) center, resulting in a Mo(IV) dihydride intermediate. Moreover, complex 2 also shows catalytic activity toward C2H2, leading to the production of polyacetylene and the extension of the vinylthioether ligand into a pendant triene chain.


Reviewers' Comments:
Reviewer #1: Remarks to the Author: The manuscript by Song, Wang and coworkers describes a molybdenum complex for catalytic hydrogenation of olefins.They found a molybdenum(II) Cp* complex which is coordinated by a bidentate sulfido phosphine ligand and acetylene (2).Protonation of the latter leads finally to a coordinated vinyl thioether.Reactivity towards H2 or C2H2 led to unusual complexes, namely a dihydrido complex and a thioether complex with a C6H7 chain at sulfur derived from two additional molecules of acetylene.Compound 2 proved to be a very good catalyst for olefin hydrogenation at mild conditions.All compounds are convincingly characterized by spectroscopic and X-ray crystallographic means and the mechanisms of compound formation supported by DFT calculations.The clearness of the manuscript is exceptionally high.
It was a pleasure to read the manuscript as it describes beautiful organometallic chemistry.Furthermore, the found catalytic activity under impressively mild conditions is of high interest.Molybdenum is quite earth abundant and thus economically interesting and it is biologically relevant (only metal of the second row of the transition metals) and therefore virtually non-toxic.
For these reasons I consider the research highly significant for researchers interesting in catalysis and the manuscript highly suitable for the high-class journal Nature Communication so that I recommend publication.
However, prior to that there are some issues/comments that need to be addressed: i) I am aware that high-resolution MS is considered a mean to support the purity of the bulk material, however I am a fan of elemental analysis.This is particularly true in case of molybdenum where molybdates tend to be quite soluble.They would not be detectable by NMR spectroscopy nor would they influence the molecular ion in MS.Thus, the authors should additionally obtain elemental analysis data for reassurance.ii) Related to the comment above, all NMR data provided in the supporting information contain minor signals that are not assigned.Particularly in 4 but also in the others.It seems that NMR data of 4 is of a material prior to isolation.Why is that?I assume that stability is an issue.Thus, macroscopic behavior of 4, and best of all Mo compounds, must be described in the manuscript and the authors have to comment on the minor signals in all spectra.iii) Why is NMR data of 1 measured at 273 K and that of 4 at 253 K and not rt?This should be mentioned in the manuscript.iv) The chemical shift difference of the two hydrido ligands in 4 is extremely high which makes one wonder whether the low-field signal is indeed a hydride.Did the authors consider other options such as an eta1-bound H2?At the very least, a possible explanation for the large difference should be discussed.v) X-ray data of 3 and 4 must be discussed by referring to literature data not only to the respective compounds in the manuscript.vi) catalysis was performed with 2 only.I assume that 4 was not investigated for stability reasons.A comment must be provided.vii) The comment on the tolerance of Mo catalyst on page 9 I cannot support.The cited literature refers to Mo(VI) from e.g.Schrock which are of course oxophilic while compound 2 in oxidation state +II is far less oxophilic.Thus, please rephase.viii) Figure 6: is the structure of Int9 possibly wrong?I cannot see how a molybdenum carbene can be formed?It is the product of acetylene insertion which gives a 7-membered metallacycle with three C-C double bonds but no metal-carbon double bond.Please comment on it.ix) Figure 6: it interesting that compound 3 is isolated in 59 % yield even though the barriers for polyacetylene formation are lower.Is 3 formed immediately or only after most acetylene has been converted to polyacetylene?Did the authors try to use stoichiometric amounts of acetylene?x) Figure 7: I find it surprising that between Int2A and Int3A no dihydride is found while later the dihydride is isolated (even 68 % yield).If no dihydride is formed, how is the alkyl formed?Do the authors consider heterolytic splitting of H2 forming H-and H+ which protonates the vinyl?While this would be similar to reactions with acetylene, I doubt it.A dihydride seems the most likely TS.A comment on what TS1A might be, must be provided.xi) Page 2: scare must be scarce.xii) Since dissociation of pyridine the authors should consider using methyl pyridines or lutidines which could possibly speed up catalysis even more.
Reviewer #2: Remarks to the Author: The manuscript by Minghui Xue and coworkers reports a molybdenum-sulfur complex 2 that catalyzes acetylene polymerization (with substantial catalyst deactivation to 3, and potentially to other species) and catalyzes alkene hydrogenation through a Mo(IV) dihydride 4. The manuscript extends recent work by the Beller and Chirik groups and others, all of which was published in specialized chemistry journals (refs 31-37).DFT-predicted mechanisms for reaction of 2 with H2 and with ethylene include rather high barriers to catalyst activation (Int2 to Int3, barrier 23.5 kcal/mol, TS3A, barrier 23.7 kcal/mol).The authors also include a speculative mechanism for alkene hydrogenation based on the DFT-predicted reaction of 2 with H2.A detailed DFT mechanism would be a better methodology.It is not clear, from the authors' writeup, that this Mo-based alkene hydrogenation catalyst will outperform existing non-precious-metal alkene hydrogenation catalysts (see the 2015 review by Chirik).In my judgement, the manuscript has a modest significance as a follow-up to refs 31-37.It would be more appropriate as a submission to the specialized chemistry journals that published refs 31-37.
Reviewer #3: Remarks to the Author: The submitted work by Wenguang Wang and coworkers is not eligible for publication in Nature Communications.The paper deals with the hydrogenation of olefins facilitated by a Mo complex.The reduction of alkenes with gaseous hydrogen is a well-established catalytic transformation and Mo is well-known for its ability to catalyze redox processes.Furthermore, the substrate scope is rather narrow and the authors merely report NMR yields for their products.Hence, the given manuscript lacks the novelty which is necessary for publication in Nat.Commun.However, the experiments including the calculations and the NMR studies were all well-performed and thus I recommend submission of the subject manuscript to a more catalysis-dedicated journal such as ChemCatChem.
Reviewer #4: Remarks to the Author: This manuscript reported a molybdenum complex for catalytic hydrogenation of various aromatic and non-activated aliphatic alkenes.As author reported, after protonated, 1-C2H2 converted to complex 2, which is the pre-catalyst for various alkenes hydrogenations.This complex can react with H2 and form Mo(IV)-dihydride 4 which is related to the proposed mechanism.Various characteristic methods as well as DFT calculations were used for the compounds' characterizations, reactions analysis and mechanism study.This is a great topic and complex 2 is an intriguing hydrogenation catalyst; however, to be suitable for publication in Nature Communications, the manuscript needs to be improved, in particular, by addressing the following key aspects: (1)The quality of characterization of Mo(IV)-dihydride 4 needs to be improved.The 1H NMR spectrum looks not pure.For SXRD, with alert A errors, and the R factor of 0.09, the H cannot be located accurately.Please provide better crystallography data or other evidence.Authors may also try deuterium experiment to locate the hydrides.Besides, the authors assigns the two hydrides, one in upfield and the other in downfield, further references and characterizations need to be provided.Moreover, for all four molybdenum complexes, please identify the solvent residue signal, water peak, and impurities in 1H-NMR.For 31P-NMR, what is the internal or external standard?For ESI-MS, please also add the whole scope of the spectra first, and then give the zoom in of founded peaks.
(2)In catalytic hydrogenation section, the authors may check the Hammett σ parameters of the substituents, the yields percentage may be related to the electron density.The effects (such as electronic/steric) on alkenes' yields need to be further analyzed to associate the mechanism.The hydrogenation reaction were only studied by small scale in NMR tube.Did the authors try bulk catalysis and isolate the products?Bulk catalysis experiment can further test the catalytic ability of the Mo complex.
(3)In the hydrogenation mechanism cycle (Fig 8), besides Mo(IV)-dihydride, is there any other intermediates observed or confirmed by any wet experiments?(4)The structure of the main-text needs to be improved.In discussion part, there are too much DFT details. Conclusions of all important experiments and future perspective need be re-written in better way.Minor Questions/Suggestions: (1)Please add percentage yields in Fig. 2., and add the temperature in the step from 1-C2H2 to 2.
(2)Merge Figure 3 and 4 together or move figure 4 into Supplementary Information.
For these reasons I consider the research highly significant for researchers interesting in catalysis and the manuscript highly suitable for the high-class journal Nature Communication so that I recommend publication.

Response:
We thank this reviewer for the positive comments and for supporting the publication of our work in Nature Communication.
However, prior to that there are some issues/comments that need to be addressed: i) I am aware that high-resolution MS is considered a mean to support the purity of the bulk material, however I am a fan of elemental analysis.This is particularly true in case of molybdenum where molybdates tend to be quite soluble.They would not be detectable by NMR spectroscopy nor would they influence the molecular ion in MS.Thus, the authors should additionally obtain elemental analysis data for reassurance.

Response:
We appreciate the suggestion from this reviewer.We have carried out elemental analysis for complexes 1-C2H2, 2, 3, and 4 during the revision process.We have included the resulting data in the "Synthesis and Characterization" section for each corresponding complex within the Supplementary Information.For more detailed information, please refer to Supplementary Information pages S4-S6.
ii) Related to the comment above, all NMR data provided in the supplementary information contain minor signals that are not assigned.Particularly in 4 but also in the others.It seems that NMR data of 4 is of a material prior to isolation.Why is that?I assume that stability is an issue.Thus, macroscopic behavior of 4, and best of all Mo compounds, must be described in the manuscript and the authors have to comment on the minor signals in all spectra.
Response: We sincerely appreciate the valuable feedback provided by this reviewer.Some minor signals in 1 H NMR spectra have been appropriately assigned in Supplementary Information.As the reviewer pointed out, compound 4 was indeed unstable in solution.We have recollected NMR spectra of 4 in C6D5Cl at 253 K. (Supplementary Fig. 28) iii) Why is NMR data of 1 measured at 273 K and that of 4 at 253 K and not rt?This should be mentioned in the manuscript.
Response: Thank you for the queries.Please see the explanations in the main text.
For 1, "At room temperature, the 1 H NMR spectrum of 1-C2H2 displayed two sets of broad resonances for the acetylenic protons at δ 10.49 and 9.48, indicating the dynamic behavior of the η 2 -C2H2 moiety binding at the Mo center. 15At 0 ℃, however, these signals became well-resolved, and the signal at δ 9.48 split into a doublet due to the coupling to the phosphorus atom (JP-H = 20 Hz). 41Further analysis of the 1 H-13 C HSQC spectrum revealed that the acetylenic protons at δ 10.49 and δ 9.48 correlate with the 13 C signals at δ 190.3 and δ 184.9, respectively." Regarding the spectroscopic properties of 4, we revised the statements as following: "The exposure of 2 (in d8-THF) to H2 (1 atm) resulted in the formation of a molybdenum dihydride compound 4, which exhibits a sharp 31 P resonance at δ 75.9. 27,50The production of 4 was initially confirmed by high-resolution mass spectroscopy (HR-MS), where a strong ionic peak at m/z = 636.1735was observed (Supplementary Fig. 14), in comparison to that of 632.1438 found for 2. When D2 was employed for the reaction, HR-MS analysis of the reaction mixture revealed an ionic peak at 640.2057 (Supplementary Fig. 15).This finding can be rationalized by the addition of two molecules of D2 to compound 2 through hydrodeuteration of the vinyl moiety and oxidative addition of D2 to the molybdenum center.At room temperature, the hydride resonances of 4 coalesced into the baseline in the 1 H NMR spectrum. 51However, upon cooling the C6D5Cl solution to 253 K, the hydride resonances appear as well-resolved peaks in the 1 H NMR spectrum.One hydride ligand shows a characteristic upfield signal at δ −4.19 as a doublet, while the signal of the other hydride is much further downfield with a chemical shift of δ 5.16 (dd). 52The 1 H-1 H COSY spectrum recorded at 253 K shows that the two sets of hydride signals correlate with each other with an extremely large 2 JH-H of 110 Hz (Fig. 3e). 51" iv) The chemical shift difference of the two hydrido ligands in 4 is extremely high which makes one wonder whether the low-field signal is indeed a hydride.Did the authors consider other options such as an eta1-bound H2?At the very least, a possible explanation for the large difference should be discussed.

Response:
We have recollected NMR spectra of 4 in C6D5Cl at room temperature.The structure of two hydrido ligands was further examined by 2D 1 H-1 H COSY spectroscopy.The literature indicates that two hydrido ligands are likely to occur in one positive and one negative position.(Dalton Trans.44, 18945−18956 (2015).)The DFT optimized structure of 4 with two hydrides is similar to the X-ray structure and the computed chemical shifts for these two hydrides are -3.7 and 2.9 ppm, respectively, which is consistent with the distinct chemical shifts observed experimentally.
According to computation, the hydride closer to the P-donor is more deshielded, i.e., has the downfield signal.This hydride is in a highly congested spot.Other than soft of being in the deshielded region of a phenyl ring (not very close though, 2.04 Å from the closest ortho H of the phenyl ring) and where we would expect a lobe from the * orbital of a P-C bond (P-H distance 2.34 Å), we have not found any unusual features of this hydride from the computational results.The Mullikan charge on both hydrides is negative (-0.30 and -0.18), with the downfield hydride being less negative.We have added the computed chemical shift to manuscript.If we move the downfield hydride toward the upfield hydride to make an η 2 -H2 complex, the computed chemical shifts are -4.0 and -0.3 ppm, respectively.v) X-ray data of 3 and 4 must be discussed by referring to literature data not only to the respective compounds in the manuscript.
Response: thanks for the queries.As the referee said, compound 4 is not stable in the solution, especially in the absence of H2.We provide a comment in the revised manuscript as: "It is important to mention that complex 4 is unstable in solution and undergoes gradual degeneration, resulting in the formation of unidentified species." We also investigated the catalytic performance of complex 4 under the same catalytic conditions of 2. We found that complex 4 also efficiently catalyzes hydrogenation of styrene: A d8-THF solution of styrene (0.2 mmol), and 4 (4 µmol, 2 mol%) contained in a J. Young NMR tube, was degassed and then exposed to H2 (1 atm) After reaction for 12 h at room temperature., the reaction produced ethylbenzene (6a) in 95% NMR yield.2. 1 H NMR (500 MHz, d8-THF) spectrum recorded for the hydrogenation of 5a catalyzed by 4.

Supplementary Figure
vii) The comment on the tolerance of Mo catalyst on page 9 I cannot support.The cited literature refers to Mo(VI) from e.g.Schrock which are of course oxophilic while compound 2 in oxidation state +II is far less oxophilic.Thus, please rephase.
Response: thanks for the careful review, we modified the statements to: "However, olefins bearing various oxygen-containing functional groups, such as ether (6t, 6u), ester (6x), amide (6w), and alcohol (6y, 6z) all underwent smooth hydrogenation.This is noteworthy as such substrates were rarely tolerated by highvalent Mo-based catalysts. 53,54" viii) Figure 6: is the structure of Int9 possibly wrong?I cannot see how a molybdenum carbene can be formed?It is the product of acetylene insertion which gives a 7membered metallacycle with three C-C double bonds but no metal-carbon double bond.
Please comment on it.

Response:
The ring expansion of the 5-membered metallacycle into a 7-membered metallacycle is a step that has many outcomes based on the potential energy surface scans.The one we drew is the immediate product from the C−C bond formation reaction, i.e., π-allyl on one side and carbene on the other side of the metallacycle based on bond lengths; this initial intermediate is metastable and isomerizes into the 7-membered metallacycle as the review suggested with a free energy barrier of 0.5 kcal/mol.On paper, this 7-membered metallacycle should have three C−C double bonds; however, the bond lengths around the metallacycle seem to suggest that the C−M bonds being double bonds, which could be due to the significant pi-back donation from the metal center.We have updated the Scheme to reflect such a change; for simplicity, we removed the initial C−C formation product and draw the 7-membered metallacycle with a delocalized circle round the ring.ix) Figure 6: it interesting that compound 3 is isolated in 59 % yield even though the barriers for polyacetylene formation are lower.Is 3 formed immediately or only after most acetylene has been converted to polyacetylene?Did the authors try to use stoichiometric amounts of acetylene?
Response: Thank you for the queries.Based on our calculations, the formation of Int3 is the slow step.When a small fraction of Int2 turns into Int3, the fast polymerization initiated by Int3 depletes C2H2 quickly.The remainder of Int2 will just turn into 3 after the C2H2 concentration drops below a certain threshold.Below is our explanation why low C2H2 concentration favors the formation of 3. From Int3, the polymerization route needs C2H2 to proceed, i.e., the rate will slow down significantly when the C2H2 concentration drops.However, the formation of 3 is intramolecular and does not get affected by this concentration drop.This hypothesis can be further demonstrated with our stoichiometric reaction, i.e., the reaction of 2 with 5 eq. of C2H2 monitored by 31 P NMR spectroscopy (see below).When the concentration of C2H2 is so low, the reaction gives 64% of 3, 14% unreacted 2 and a few stalled intermediates (which might be Int2).When the polymerization reaction under 1 atm of C2H2 was monitored by 31 P NMR experiment, (see Figure SS1 added), the formation of 3 was observed in 20 minutes.It is worth noting that in this NMR tube, reaction the diffusion of C2H2 into solution is slow without stirring.Consequently, the initial C2H2 concentration is low, which favors the formation of 3. As such, the immediate formation of 3 is expected and observed.
Under N2 atmosphere, in an NMR tube fitted with a rubber stopper, 2 (15mg, 0.01 mmol) and 1,3,5-trimethoxybenzene (1.68 mg, 0.01 mmol) were dissolved in d8-THF (0.6 mL).The tube was taken out from the glovebox and injected C2H2 (1.23 mL, 0.05 mmol) using a syringe at 25℃ (Vm = 24.5 L/mol).After the indicated time, we monitored changes in 31 P NMR and 1 H NMR spectra, respectively.x) Figure 7: I find it surprising that between Int2A and Int3A no dihydride is found while later the dihydride is isolated (even 68 % yield).If no dihydride is formed, how is the alkyl formed?Do the authors consider heterolytic splitting of H2 forming H-and H+ which protonates the vinyl?While this would be similar to reactions with acetylene, I doubt it.A dihydride seems the most likely TS.A comment on what TS1A might be, must be provided.

Response:
Indeed, what we found based on the potential energy surface scan is the heterolytic splitting of the coordinated H2 between the metal and the olefin.The following structural description of TS1A has been added to the discussion, "In TS1A the distance between the two hydrogen atoms originated from H2 is 1.51 Å, shorter than that of a typical dihydride; TS1A is most consistent with the heterolytic splitting of a coordinated H2 between the olefin carbon and the metal center, i.e., deprotonation of the H2 ligand by the alkene ligand."xi) Page 2: scare must be scarce.
Response: Thanks for the careful review.It has been changed to "scarce".
xii) Since dissociation of pyridine the authors should consider using methyl pyridines or lutidines which could possibly speed up catalysis even more.

Response:
We appreciate this reviewer for the insightful comments.
Similar to the reaction with pyridinium salts, the molybdenum acetylene compound also reacts with 2,6-lutidinium salts.According to 31 P NMR (Supplementary Fig. 7), the reaction also produced the analogue vinyl complex 2'.
Under the identical catalytic conditions of 2, the catalytic performances of 2' in the hydrogenation of styrene were compared.(please see Supplementary Fig. 8 and Fig. 9) Synthesis of complexes 2 and 2′: Under N2 atmosphere, a green solution of 1-C2H2 (30 mg, 0.054 mmol) in 3 mL of THF was cooled to -20 ℃.The resulting solution was treated with 2,6-lutidinium salts (0.053 mmol) in 2 mL of THF, which caused a change in the solution color from green to brown.After the removal of the solvent under vacuum, the solid was washed with hexane (5-10 mL).The corresponding complexes 2 and 2′ were precipitated and separately isolated as brown powder.
Hydrogenation of 5a catalyzed by 2 and 2′: In an N2-filled glovebox, to three J. Young NMR tubes charged separately with catalyst 2 and 2' (0.004 mmol), 5a (0.2 mmol) in d8-THF (0.6 mL), was added 1,3,5-trimethoxybenzene (11.2 mg, 0.067 mmol) as the internal standard.The tube was taken out from the glovebox and immersed in a liquid nitrogen bath, and gently degassed under a vacuum.The solution was then warmed up to room temperature and pressurized with H2 gas (1 atm).After reaction at room temperature for 3 h, the solution was analyzed by 1 H NMR to determine the yield of 6a. and potentially to other species) and catalyzes alkene hydrogenation through a Mo(IV) dihydride 4. The manuscript extends recent work by the Beller and Chirik groups and others, all of which was published in specialized chemistry journals (refs 31-37).DFTpredicted mechanisms for reaction of 2 with H2 and with ethylene include rather high barriers to catalyst activation (Int2 to Int3, barrier 23.5 kcal/mol, TS3A, barrier 23.7 kcal/mol).The authors also include a speculative mechanism for alkene hydrogenation based on the DFT-predicted reaction of 2 with H2.A detailed DFT mechanism would be a better methodology.It is not clear, from the authors' writeup, that this Mo-based alkene hydrogenation catalyst will outperform existing non-precious-metal alkene hydrogenation catalysts (see the 2015 review by Chirik).In my judgement, the manuscript has a modest significance as a follow-up to refs 31-37.It would be more appropriate as a submission to the specialized chemistry journals that published refs 31-37.

Response:
The main source of error for the computed energetics is the imperfect solvation, especially when the catalyst is cationic and when the reaction involves charge redistribution.Our catalyst is cationic and both steps in question involve significant charge redistribution, i.e., oxidative cycloaddition from Int2 to Int3 and reductive elimination from Int3A to Int4A (even the step from Int2A to Int3A involves charge redistribution).It is well known that the solvation issue is significant for this kind of systems; so, a certain degree of errors in computed free energy is expected.In addition, the barriers of 23.5 kcal mol -1 (from Int2 to Int3) is also slightly inflated because the default standard state in Gaussian for gaseous starting material is p 0 = 1, but the reaction we are considering is in solution, i.e., the standard state should be [X]0 = 1 M instead.
Response: Thanks for acknowledging the technical quality of our work.To further demonstrate the capability of our catalyst, we have expanded the substrate scope and performed two large-scale late-stage hydrogenation of a natural product with isolated yields.
(1)The quality of characterization of Mo(IV)-dihydride 4 needs to be improved.The 1H NMR spectrum looks not pure.For SXRD, with alert A errors, and the R factor of 0.09, the H cannot be located accurately.Please provide better crystallography data or other evidence.Authors may also try deuterium experiment to locate the hydrides.
Besides, the authors assigns the two hydrides, one in upfield and the other in downfield, further references and characterizations need to be provided.Moreover, for all four molybdenum complexes, please identify the solvent residue signal, water peak, and impurities in 1H-NMR.For 31P-NMR, what is the internal or external standard?For ESI-MS, please also add the whole scope of the spectra first, and then give the zoom in of founded peaks.

Response:
We agree with the reviewer's suggestion.
(i) For SXRD, we have corrected the alert A error, and the R factor of 0.08.
(ii) At room temperature, the hydride resonances of 4 coalesced into the baseline in the 1 H NMR spectrum.We have recollected NMR spectra of 4 in C6D5Cl at 253K.The structure of two hydrido ligands was further examined by 2D 1 H-1 H COSY spectroscopy.The literature indicates that two hydrido ligands are likely to occur in one positive and one negative position.(Dalton Trans.44, 18945−18956 (2015).)The attempts to locate the hydrides by deuterium experiment at 253K are always unsuccessful because compound 4 is not stable in the solution.Especially, the NMR deuterium spectrum exhibits a decrease in the signal-to-noise ratio and a weakening of the signal under 253K.In addition, The DFT optimized structure of 4 with two hydrides is similar to the X-ray structure and the computed chemical shifts for these two hydrides are -3.7 and 2.9 ppm, respectively, which is consistent with the distinct chemical shifts observed experimentally.According to computation, the hydride closer to the P-donor is more deshielded, i.e., has the downfield signal.This hydride is in a highly congested spot.
Other than sort of being in the deshielded region of a phenyl ring (not very close though, 2.04 Å from the closest ortho H of the phenyl ring) and where we would expect a lobe from the * orbital of a P-C bond (P-H distance 2.34 Å), we have not found any unusual features of this hydride from the computational results.The related results have been added into the revised manuscript, please see: "To evaluate the influence of electronic variation in the substituent on the reaction rate, kinetic studies were performed on hydrogenation of the para-substituted styrene derivatives (p-X-styrene, 5a-5g).The reaction progress was monitored using 1 H NMR spectroscopy, revealing a linear reaction profile within the initial 4 hours (Supplementary Fig. 10).The reaction rate was found to strongly depend on the electronic nature of the para-substituent.The kinetic data are correlated with the standard Hammet σpara values, 55 resulting in a negative slope of ρ = -0.64 (Fig. 3g).The small absolute value of the Hammett electronic parameter suggests that the reaction site in the turnover-limiting transition state is more remote than the benzylic carbon from the para-X-group, ruling out the insertion step being the turnover-limiting. 56The small negative ρ is consistent with the reductive elimination step being turnover-limiting, where the terminal carbon of the styrene substrate is involved." ii) We have added bulk catalytic hydrogenation of Allylestrenol (5ad) with general reaction condition.The isolated product 6ad and d2-6ad were identified by 1 H NMR and 2 H NMR. (see Supplementary Fig. 59 and Fig. 60).
In an N2-filled glovebox, a flame-dried Schlenk tube was charged with catalyst 2 (149.6 mg, 2 mol%), 5ad (601.0 mg, 2.0 mmol) in d8-THF (3.0 mL).The tube was taken out from the glovebox and immersed in a liquid nitrogen bath, and gently degassed under vacuum.The solution was then warmed up to room temperature and pressurized with H2 gas (1 atm).After reaction at room temperature for 12 h, the mixture was evaporated to dryness, and the crude product was purified by column chromatography on silica gel eluting with petroleum ether/EtOAc (10: 1) to give the corresponding product 6ad (white solid, 540.6 mg, 90%).
Following the procedure of reaction with H2, 6ad also reacts with D2.After the reaction, the deuterated product d2-6ad (white solid, 98.3 mg, 91%) was obtained.
(  16187−16189 (2008).;Eur.J. Inorg.Chem.2011, 141−149 (2011).) (4)The structure of the main-text needs to be improved.In discussion part, there are too much DFT details. Conclusions of all important experiments and future perspective need be re-written in better way.

Response:
We have re-written this part based on the suggestions from this reviewer and reviewer 1.
Regarding the conclusions and future perspectives, we reorganized the conclusion parts: "We have demonstrated the protonation of a molybdenum(II)-acetylene complex using a pyridinium salt, resulting in the formation of a novel cationic Mo(II)-vinylthioether complex.This complex exhibits efficient catalytic activity for hydrogenation of various aromatic and aliphatic alkenes at ambient temperature.The reaction exhibits excellent substrate compatibility and high functional group tolerance without the need for any additives.Mechanism studies reveal that the catalysis involves a Mo(IV) dihydride intermediate arising from an uncommon oxidative addition of H2 to the cationic Mo(II) center.Additionally, we found that complex 2 displayed intriguing reactivity toward C2H2, causing the vinylthioether ligand to undergo chain propagation and yield a dangling triene chain.Our future studies will focus on modifying the coordination sphere of [Cp*Mo(1,2-Ph2PC6H4SR)] to enhance the catalytic performance in the transformation of C2H2 and substituted alkynes." Minor Questions/Suggestions: (1) Please add percentage yields in Fig. 2., and add the temperature in the step from 1-C2H2 to 2.
Response: Thanks for the careful review.In the revised Fig. 2, the yields have been provided for each step, and the reaction temperature has been noted for the conversion of 1-C2H2 to 2.
(2) Merge Figure 3 and 4 together or move figure 4 into Supplementary Information.
Response: Figure 3 and 4 has been combined, please see Figure 3 in the revised manuscript.
)In the hydrogenation mechanism cycle (Fig 8), besides Mo(IV)-dihydride, is there any other intermediates observed or confirmed by any wet experiments?Response: There are no other intermediates were observed in catalytic hydrogenation by wet 31 P NMR.The X-ray structure and NMR spectrum of complex 4 indicate that the cleavage of H2 at the molybdenum center without intermediate.The related examples of oxidative addition of H2 at Mo center are reported.(J.Am.Chem.Soc.130,